// 合并二叉树
var mergeTrees = function(root1, root2) {
    if(root1==null && root2==null) return null;
    if(root1==null) return root2;
    if(root2==null) return root1;
    root1.val+=root2.val;
    root1.left=mergeTrees(root1.left,root2.left);
    root1.right=mergeTrees(root1.right,root2.right);
    return root1;
};
// 官方题解
var mergeTrees = function (root1, root2) {
    const preOrder = (root1, root2) => {
        if (!root1)
            return root2
        if (!root2)
            return root1;
        root1.val += root2.val;
        root1.left = preOrder(root1.left, root2.left);
        root1.right = preOrder(root1.right, root2.right);
        return root1;
    }
    return preOrder(root1, root2);
};